1. Field of the Invention
The present invention relates generally to actuators having safety mechanisms for locking the axial position of the actuator piston with respect to the actuator cylinder. More particularly, the invention relates to a simple and reliable fluid-powered BOP actuator which mechanically locks the ram secured to the piston in multiple closed positions to maintain the flow path through the BOP sealed off even though fluid pressure to the actuator is interrupted.
2. Description of the Background
Blowout preventors (BOPs) are used in oil field operations to close off the flow path either through a tubular string or through an annulus between the tubular string and the BOP body. The BOP actuators are typically fluid-powered to effect both closing and opening of the ram shafts which each axially move within a respective actuator cylinder. Actuators on opposing sides of the BOP body typically press ram blocks together simultaneously to close off the desired flow path through the BOP. BOPs are often actuated to seal off a well during an emergency or during well shutdown operations prior to anticipated inclement weather at offshore installations.
Since the supply of pressurized fluid to the BOP may become interrupted after closing of the BOP, actuators have been designed with the various types of safety mechanisms to lock or fix the ram shaft in its closed position and thereby prevent inadvertent release of the closing pressure on the ram. Since BOPs are often located in sub-sea or other remote environments, BOP actuators and lock mechanisms associated therewith must be highly reliable. Those skilled in the art appreciate the significant safety and environmental risks resulting from the inadvertent opening of the BOP rams, and accordingly both primary and backup systems have been devised to prevent this occurrence.
Prior art BOP ram actuators were initially equipped with locking mechanisms which fixed the position of the piston at a single axial location within the actuator cylinder, and thus fixed the ram in a single closed position. An example of BOP actuator with such a lock mechanism is disclosed in U.S. Pat. No. 3,242,826. The lock mechanism includes expandable locking segments which engage an annular recess in the cylinder wall and thereby lock the ram shaft position with respect to the actuator cylinder. A single position lock mechanism has several drawbacks, however, since wear of the ram block components requires continual adjustment of the lock mechanism to achieve a lock condition and thereby seal off the flow path through the BOP at a reliable sealing pressure.
Another problem associated with BOP lock mechanisms concerns the goal of minimizing the frictional drag during normal operation of the actuator between the locking elements which axially fix the ram shaft within the actuator cylinder, while simultaneously providing a locking mechanism which cannot be overcome and thereby allow opening of the ram shaft before the locking mechanism is intentionally disengaged. U.S. Pat. No. 4,519,571 discloses actuator locking elements which are wedged outwardly along a locking shoulder of the cylinder as the ram is moved towards its closed position. Unless fluid pressure is available to maintain the locking elements in their closed position, however, the locking elements can be disengaged by overcoming frictional forces between the locking components. The above patent discloses a locking mechanism capable of achieving a lock at various axial locations, although within a relatively narrow range. While locking mechanism adjustment for ram block component wear is minimized, the same BOP actuators cannot be reliably used with different ram blocks for locking about different sized pipes passing through the BOP. Moreover, the reliability of the locking mechanism is dependent on maintaining fluid pressure to the BOP to keep the locking elements in their closed position.
A multiple-position locking mechanism commercially offered by Hydril allows BOP actuator lock at various positions, so that the ram block may seal off with a high closing force although the seals on the ram block wear. The ram is closed by hydraulic pressure, and is maintained closed by a unidirectional clutch, lock nut, and threaded tail rod assembly. Forces attempting to open the ram are transmitted through the tail rod to the lock nut and then to the locked clutch plates. Hydraulic pressure is used to disengage the clutch plates and thereby permit the lock nut to rotate on the tail rod to allow the ram shaft to open. This locking mechanism is complex and is thus considered costly to maintain. The locking mechanism undesirably increases the diameter of the BOP assembly since the tail rod and clutch assembly are axially aligned with and extend from the actuator cylinder at a location opposite the ram blocks.
Each of the prior art BOP actuators with locking mechanisms thus has significant disadvantages relating to reliability, versatility, and/or cost. Single or dual position locking mechanisms do not provide the desired lock closing pressure once ram seals wear. Locking devices which effect locking at multiple axial locations along a narrow range cannot be used with interchangeable ram blocks to seal off various diameter tubular passing through the BOPs. Since fluid pressure to the BOPs may become interrupted either during an emergency or during a long delay after a BOP is closed, actuator locking mechanisms which require fluid pressure to ensure locked engagement of the ram shaft within the cylinder do not offer the desired high reliability. Locking mechanisms which rely on frictional engagement of components are undesirable since the lock device may fail if sufficient opening pressure is applied to the actuator ram. A locking mechanism which includes a unidirectional clutch, lock nut, and threaded tail rod assembly is costly and undesirably increases the size and complexity of the BOP. Most prior art locking mechanism cannot practically be incorporated into existing BOP actuators, and accordingly high replacement costs are required to provide a BOP with a versatile actuator locking device.
The disadvantages of the prior art are overcome by the present invention, and an improved blowout preventor actuator is hereinafter disclosed which simply and reliably locks the position of a BOP ram at multiple locations extending axially over a relatively long range. The actuator need not be repeatedly readjusted for ram block wear, and the same actuator may be used with replaceable ram blocks to close off various diameter flow passageways through the BOP.